The Heithaus test, which models asphalt explicitly as a colloidal system, was developed in the early 1960's by J. J. Heithaus to study compatibility characteristics of petroleum residua used in the roofing industry. Since then, the Heithaus test has found use in the paving industry as a method to study rutting propensity and oxidative age hardening. The original method, which suffered from operator dependency and poor data repeatability, has recently been automated. An automated Heithaus titration (AHT) test has been developed based on light transmitting/scattering detection of the onset of flocculation using ultraviolet (UV)-visible spectrophotometry. The AHT test has been found to significantly reduce operator dependency and improve data repeatability, in some cases, by an order of magnitude. As a result of the improved repeatability of data, Heithaus parameters are found to measure physical properties that relate to rheological properties of asphalt.
Historically, asphalts have been classified into gel-type asphalts and sol-type asphalts. Gel-type asphalts usually are characterized by non-Newtonian rheological behavior, relatively low variation of viscosity with temperature, and low ductility. Sol-type asphalts exhibit more Newtonian rheological behavior, are highly temperature susceptible, and are more ductile. The two classifications represent extremes; most asphalts are of an intermediate nature. Sol-type asphalts have also been designated as compatible asphalts, while gel-type asphalts have been designated as incompatible asphalts.
The terms “compatible” and “incompatible” (or even sol and gel) arose from what became known as the colloidal model of asphalt structure and often are used as general terms to relate “self-compatibility” and “self-incompatibility”. This model considers asphalts to be dispersions of what are termed “micelles,” consisting of polar, aromatic molecules in viscous oils. In the model, the degree to which the so-called “micelles” form extended gel structures (which can be broken up by heat and shear) determines the relative degree of compatibility. In a compatible asphalt, the dispersed materials are believed to be well peptized by the solvent, either because the dispersed materials are small in amount and/or tend not to form strong associations, and/or because the solvent effectively disperses the “micelles.” In an incompatible asphalt, associations of dispersed materials presumably are more extensive and are not so efficiently peptized by the solvent.
The colloidal model has been subjected to much criticism in recent years. The principal objection is that there is no evidence for “micellar” structures, either classical or inverse, in asphalts. The term “micelle,” which implies the existence of a separate phase with distinct boundaries, may in fact be inappropriate. More recently, a different microstructural model of asphalt structure had been proposed. Even this has now been refined by the present inventors. In the model, associations of polar, aromatic molecules of varying sizes are considered to be dispersed in a solvent moiety composed of less polar, relatively small molecules. No distinct phase boundaries are believed to be present. Regardless of the validity of the model, though, the concept of compatibility as a measure of mutual miscibility of different chemical components of asphalts is still useful. Compatible asphalts differ from incompatible asphalts in their physical properties and therefore may be expected to behave differently in pavements. Changes in the degree of compatibility often have opposing effects on important performance related properties. For example, a change that may result in better rutting resistance may also result in more embrittlement resulting from oxidative age hardening. Thus, compromises in compatibility can be viewed as necessary for optimum overall pavement performance.
Asphaltenes are solid materials that precipitate when asphalts are treated with solvents such as n-pentane, n-hexane, n-heptane, iso-octane, etc. Maltenes are the components of asphalts not precipitated by the above alkane solvents. Asphaltenes are more aromatic than maltenes and contain more heteroatoms. Thus intermolecular interactions are likely more extensive in asphaltenes than in maltenes. This may be reflected in the greater molecular weights of asphaltenes compared with maltenes. In the colloidal model of asphalt structure, asphaltenes are believed to correspond to the dispersed materials and maltenes to the solvent. Therefore, asphaltenes may be mainly responsible for the internal structure of asphalts and may dominate many physical properties. Thus the amount of asphaltenes in an asphalt could be one measure of compatibility. Compatible asphalts may have smaller amounts of asphaltenes than incompatible asphalts. The ease with which asphaltenes are dispersed may be dependent on their peptizability and on the dispersing power of maltenes. Oxidative aging of an asphalt could be predicted to influence compatibility by formation of polar molecules, which may result in more extensive molecular associations, but also may result in a better solvent.
The best known measurement of compatibility of asphalts that takes all the above factors into account is the Heithaus test. Heithaus observed that for straight-run asphalts, measuring asphaltene contents provided a reasonably good estimate of compatibility. Perhaps surprisingly, in blended asphalts from different sources (composite asphalts or asphaltic composites), weight-averaging asphaltene contents did not provide reliable estimates of compatibility. It thus was viewed as necessary to test each blend and develop a different method that took into consideration factors other than asphaltene content. In Heithaus' original “classical” test, solutions of various concentrations containing different weights of asphalt (Wa) were dissolved in a constant volume of solvent (VS), e.g., toluene or benzene, were titrated with normal alkane solvents, including, e.g., n-heptane, until flocculation (asphaltene precipitation) was observed. Flocculation was detected by spotting a drop of the solution onto filter paper, to permit the resulting phase separation of precipitated material from material remaining in solution to be observed. This was done by a direct observation or through the use of a microscope. The volume of titrant (VT) required to initiate flocculation in each solution was then used to determine flocculation ratios (FR), calculated as FR=VS/(VS+VT). Values of flocculation ratios were plotted versus dilution concentration (C), calculated as C=Wa/(VS+VT) and a best fit straight line connecting the points was extrapolated to the x- and y-axes. The x and y intercepts determined from the extrapolation, referred to as the dilution concentration minimum (Cmin) and the flocculation ratio maximum (FRmax), respectively, were used to calculate three Heithaus parameters, defined below.
The theoretical significance of the quantity Cmin was that it represented the quantity of titrant (n-heptane for the classical method) that would be just enough to cause asphaltene precipitation in the neat asphalt, undissolved in toluene, assuming it would be possible to do so. FRmax represented a measure of the solubility parameter, δ, which may be measured in Hildebrand units, H, at which asphaltene flocculation occurred in the asphalt as a whole. Thus, the Heithaus method measured some fundamental properties of asphalts and blends that asphaltene concentration values did not measure.
In the original “classical” test, the Heithaus parameters were: pa=1−FRmax, which measured the peptizability of the asphaltene fraction; po=FRmax(Cmin−1+1), which measured the solvent power of the maltene fraction, and P=po/(1−pa), which measured the overall compatibility of the asphalt. Larger values of pa, po, and P represented peptizable asphaltenes, maltenes that were a good solvent, and a compatible asphalt overall. Smaller values of pa, po, and P represented the reverse. Interestingly, the pa and po values did not necessarily vary directly with one another among asphalts. An asphalt may be composed of asphaltenes that are not readily peptizable, but which are dispersed in maltenes that have good solvent characteristics, or the reverse.